CODEWITHSUNDEEP
c++eigeneigen3
bremen_matt
bremen_matt

Reputation: 7349

Referencing Eigen Matrices

I expected that when I create an Eigen matrix and set it equal to another matrix, that I would see reference semantics. Specifically, I expected the output of this 

#include <iostream>
#include "Eigen/Dense"

using Eigen::MatrixXd;
using Eigen::VectorXd;

int main() {

    MatrixXd A = MatrixXd::Identity(1, 2);
    MatrixXd B = A;
    A(0, 0) = 4;
    std::cout << "A: " << A << std::endl;
    std::cout << "B: " << B << std::endl;
    return 0;
}

to be 

A: 4 0
B: 4 0

but I actually get 

A: 4 0
B: 1 0

So I guess Eigen is using copy semantics for these constructors. But if I explicitly reference the input (like in a class constructor), I still seem to be getting copy semantics. Specifically, the output of 

#include <iostream>
#include "Eigen/Dense"

using Eigen::MatrixXd;
using Eigen::VectorXd;

class Holder {

public:
    MatrixXd mat;

    Holder(MatrixXd &A) : mat(A) {
    }
};

int main() {

    MatrixXd A = MatrixXd::Identity(1, 2);
    Holder C(A);
    A(0, 0) = 4;
    std::cout << "A: " << A << std::endl;
    std::cout << "C: " << C.mat << std::endl;
    return 0;
}

is 

A: 4 0
C: 1 0

So the question is... why does it seem like Eigen is copying the matrices instead of referencing them?

Also, in the end I would like to define a single matrix (e.g. A in the above example), then create many instances of the Holder class, where each instance has a reference to the same matrix. Specifically, I want the behavior that modifying A modifies the matrices in the Holder classes. The catch is that I cannot modify the Holder class itself. So, for instance, I could not change the Holder class to save a pointer to A instead of A itself. How would I accomplish that?

I have scoured the Eigen docs, but can't seem to find the explanation.

Upvotes: 0

Views: 694

Answers (1)

amc176
amc176

Reputation: 1544

Eigen is copying because you are asking it to copy. If you want to have a reference to a Matrix, you have to specify it using the correct syntax. In C++, reference variables are specified with the & character:

MatrixXd A = MatrixXd::Identity(1, 2); // this is an object
MatrixXd& B = A; // this is a reference to an object (notice the &)

Upvotes: 1

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